Taking the Fifth is a basic Constitutional right and not evidence of guilt. Republicans have made clear that they would like Fauci to be prosecuted. His pardon doesn’t protect him from state prosecutions or federal prosecutions for anything that happened after the date of the pardon. When you have an administration and a party that abuses justice with lawfare, it makes more sense for people to plead the fifth.
I’d be nice for Fauci to answer questions in case we could learn anything, or be reminded of something, useful or interesting about what happened during Covid. But, the Republican Senators weren’t there to learn, they were there to grandstand, humiliate, and accuse.
Taking the Fifth is basic defense attorney advice that any would give Fauci in that situation.